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Fragile X-syndrome is a common cause of mental retardation that is inherited as an X-linked dominant disorder with reduced penetrance.Fragile X syndrome has been shown to be caused by an unstable CGG repeat within the fragile X mental retardation-1(FM$R1)gene.The risk of having an affected child is directly related to the number of maternal repeats,with sequentially increasing probabilities of these alleles converting to full mutations as they are transmitted to subsequent generations.Advances have led to highly accurate laboratory diagnosis of both carrier and affected individuals as well as markedly improved prenatal diagnosis.In addition,a previously unrecognized class of mutation,later found responsible for several other important genetic diseases,has emerged. |
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